Ultrasonic measurement apparatus, ultrasonic diagnostic apparatus, and ultrasonic measurement method

ABSTRACT

An ultrasonic measurement apparatus includes a transmission processing unit that performs processing for transmitting an ultrasonic wave at a given transmission angle, a reception processing unit that performs reception processing of an ultrasonic echo with respect to a transmitted ultrasonic wave, and a processing unit that performs processing with respect to a reception signal from the reception processing unit. The processing unit obtains a plurality of first resolution signals by synthesizing a plurality of the reception signals based on a first beamforming coefficient, and obtains a second beamforming coefficient for synthesizing a second resolution signal from the plurality of first resolution signals based on whether a signal processing target point belongs to a plane wave propagation region or belongs to a spherical wave propagation region.

BACKGROUND

1. Technical Field

The present invention relates to an ultrasonic measurement apparatus, anultrasonic diagnostic apparatus, an ultrasonic measurement method, andthe like.

2. Related Art

In ultrasonic measurement apparatuses (ultrasonic diagnosticapparatuses), synthetic aperture processing in which focusing can beformed in overall region of an observation region is adopted thereto inorder to obtain favorable resolution throughout a wide region in anultrasonic image. According to the aforementioned technique, focusingcan be formed with the fewer number of times of transmissions andreceptions, and processing is performed so as to be able to attain ahigh frame rate and high resolution.

Among methods of the synthetic aperture processing, there is knowntechnique in which plane waves are used as transmission waves. Forexample, JP-A-2003-220059 (Patent Literature 1) discloses technique ofperforming the synthetic aperture processing in consideration ofdiffraction characteristics of ultrasonic waves while using the planewaves as transmission waves.

Patent Literature 1 also discloses technique in which improvement ofresolution is realized by introducing an adaptive weight with respect toeach signal when executing signal synthesis of the synthetic apertureprocessing. For example, according to Iben Kraglund Holfort et al,“Adaptive Receive and Transmit Apodization for Synthetic ApertureUltrasound Imaging”, ULTSYM.2009 (Non-Patent Literature 1), improvementof resolution is realized by executing adaptive beamforming with respectto an synthetic aperture technique in which a transmission method usingspherical waves is adopted, and performing weighting addition withrespect to low-resolution signals obtained through each transmission.Meanwhile, according to Andreas Austeng et al, “Coherent Plane-WaveCompounding and Minimum Variance Beamforming” ULTSYM.2011 (Non-PatentLiterature 2) which is applied with Non-Patent Literature 1, improvementof resolution is realized by executing the adaptive beamforming withrespect to the synthetic aperture technique in which anothertransmission method using plane waves is adopted.

In technique disclosed in Non-Patent Literature 2, weights according toa minimum variance beamforming method (hereinafter, referred to as theMVB method) are introduced with respect to low-resolution signals whichare obtained through multiple times of transmissions and receptions.However, in the technique, an improvement effect of resolution can beobtained in only a region in which each transmission wave is propagatedand superimposed as a plane wave.

The region in which the plane waves are superimposed varies inaccordance with a depth (an observation depth) of a signal processingtarget point with respect to a transmission scanning angle, and when thetransmission scanning angle is increased or the observation depth isdeepened, there is generation of a signal of a region in which a planewave propagation model does not come into existence. If the MVB methodis executed in such a case, there is an influence caused by a signalother than the plane wave (for example, a spherical wave) so that theimprovement effect of resolution cannot be obtained, thereby leading toan occurrence of deterioration of an image. In other words, there is adisadvantage of limitation on a region in which the improvement effectof resolution according to the MVB method can be obtained.

SUMMARY

An advantage of some aspects of the invention is to provide anultrasonic measurement apparatus, an ultrasonic diagnostic apparatus,and an ultrasonic measurement method in which an adaptive weight appliedto synthesizing processing is suitably calculated by using adetermination result regarding where a signal processing target pointexists in any one of a plane wave propagation region and a sphericalwave propagation region so as to obtain an effect of adaptivebeamforming throughout a wide observation region.

An aspect of the invention relates to an ultrasonic measurementapparatus including a transmission processing unit that performsprocessing for transmitting an ultrasonic wave at a given transmissionangle, a reception processing unit that performs reception processing ofan ultrasonic echo with respect to a transmitted ultrasonic wave, and aprocessing unit that performs processing with respect to a receptionsignal from the reception processing unit. The processing unit obtains aplurality of first resolution signals by synthesizing a plurality of thereception signals based on a first beamforming coefficient, and obtainsa second beamforming coefficient for synthesizing a second resolutionsignal having high resolution compared to the first resolution signalfrom the plurality of first resolution signals based on whether a signalprocessing target point belongs to a plane wave propagation region inwhich the ultrasonic wave is propagated as a plane wave or belongs to aspherical wave propagation region in which the ultrasonic wave ispropagated as a spherical wave.

According to the aspect of the invention, the second beamformingcoefficient is obtained based on where the signal processing targetpoint exists in any one of the plane wave propagation region and thespherical wave propagation region. Therefore, adaptive beamforming canbe performed by using a result of region discrimination processing, andthus, it is possible to obtain the effect of adaptive beamformingthroughout a wide observation region.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the processing unit may select a coefficient computationfirst resolution signal from the plurality of first resolution signalsbased on the result of the region discrimination processing regardingwhether the signal processing target point belongs to the plane wavepropagation region or belongs to the spherical wave propagation region,and may obtain the second beamforming coefficient based on the selectedcoefficient computation first resolution signal.

With this configuration, the coefficient computation first resolutionsignal is selected from a plurality of first resolution images, and theselected coefficient computation first resolution signal is used for theprocessing. Thus, it is possible to suitably obtain the secondbeamforming coefficient.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the processing unit may synthesize the selected coefficientcomputation first resolution signal and may generate the secondresolution signal based on the obtained second beamforming coefficient.

With this configuration, it is possible to use the coefficientcomputation first resolution signal as a target of the synthesizingprocessing using the second beamforming coefficient.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the plane wave propagation region and the spherical wavepropagation region may be regions different from each other inaccordance with the transmission angle of the ultrasonic wave in thetransmission processing unit.

With this configuration, it is possible to decide the plane wavepropagation region and the spherical wave propagation region inaccordance with the transmission angle of an ultrasonic wave.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the processing unit may perform region discriminationprocessing regarding where the signal processing target point exists inany one of the plane wave propagation region and the spherical wavepropagation region.

With this configuration, it is possible to perform the regiondiscrimination processing.

In the ultrasonic measurement apparatus according to the aspect of theinvention, a storage unit that stores table data in which informationindicating where the signal processing target point exists in any one ofthe plane wave propagation region and the spherical wave propagationregion is caused to correspond to the given signal processing targetpoint for each transmission angle of a plurality of the transmissionangles of the ultrasonic wave from the transmission processing unit maybe included further. The processing unit may perform the regiondiscrimination processing based on the table data.

With this configuration, it is possible to perform the regiondiscrimination processing based on the stored table data.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the processing unit may perform the region discriminationprocessing based on a first direction in which a first aperture endamong apertures respectively provided with a plurality of ultrasonictransducers transmitting the ultrasonic wave leads to the signalprocessing target point, a second direction in which a second apertureend different from the first aperture end among the apertures leads tothe signal processing target point, and the transmission angle of theultrasonic wave.

With this configuration, it is possible to perform the regiondiscrimination processing by using information of various directions.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the transmission processing unit may perform processing fortransmitting first to Kth (K is an integer equal to or greater than 2)ultrasonic waves at first to Kth transmission angles. The processingunit may perform the region discrimination processing for discriminatingwhere the signal processing target point exists in any one of an ith (iis an integer of l≦i≦K) plane wave propagation region and an ithspherical wave propagation region corresponding to an ith ultrasonicwave based on an ith transmission angle and a position of the signalprocessing target point.

With this configuration, it is possible to perform the regiondiscrimination processing with respect to each step of transmissionprocessing when the transmission processing is performed multiple timesat the varied transmission angles.

In the ultrasonic measurement apparatus according to the aspect of theinvention, the reception processing unit may perform receptionprocessing of the ultrasonic echo with respect to a transmittedultrasonic wave in first to Nth ultrasonic transducers. The processingunit may perform phasing processing with respect to first to Nthreception signals respectively corresponding to the first to Nthultrasonic transducers, may synthesize the first to Nth receptionsignals after phasing processing based on the first beamformingcoefficient, and may generate the first resolution signal.

With this configuration, it is possible to perform processing forobtaining a synthetic signal (the first resolution signal) bysynthesizing a reception signal which is acquired from each elementamong a plurality of elements, and to perform processing for obtainingoutput signal (the second resolution signal) by synthesizing a syntheticsignal which is obtained in response to each step of the transmissionprocessing of a plurality of steps among the transmission processing.

Another aspect of the invention relates to an ultrasonic diagnosticapparatus including the ultrasonic measurement apparatus describedabove.

Still another aspect of the invention relates to an ultrasonicmeasurement method including processing for transmitting an ultrasonicwave to a target object, performing reception processing of anultrasonic echo with respect to a transmitted ultrasonic wave, obtaininga plurality of first resolution signals by synthesizing a plurality ofreception signals received through the reception processing based on afirst beamforming coefficient, and obtaining a second beamformingcoefficient for synthesizing a second resolution signal having highresolution compared to the first resolution signal from the plurality offirst resolution signals based on whether a signal processing targetpoint belongs to a plane wave propagation region in which the ultrasonicwave is propagated as a plane wave or belongs to spherical wavepropagation region in which the ultrasonic wave is propagated as aspherical wave.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIGS. 1A and 1B are diagrams illustrating reception timing of areflected wave which is different from each other in accordance with aposition of a signal processing target point.

FIG. 2 is an example in which signals from the signal processing targetpoints different from each other are received at the same timing.

FIG. 3 is an explanatory diagram of phasing processing.

FIG. 4 is a diagram illustrating synthesizing processing of the Nreception signals which are received by N elements.

FIG. 5 is an explanatory diagram of transmissions performed multipletimes at varied transmission angles.

FIG. 6 is a diagram illustrating synthesizing processing of K×Nreception signals which are received by the N elements through K timesof transmissions of transmission waves.

FIG. 7 is a sound field model at the time of transmitting a plane wave.

FIGS. 8A and 8B are explanatory diagrams of a relationship among a planewave propagation region, a depth of the signal processing target point,and a transmission scanning angle.

FIG. 9 is a diagram illustrating a relationship between the transmissionangle and sound pressure.

FIG. 10A is a synthesis example when adaptive beamforming is notapplied, and FIG. 10B is another synthesis example when adaptivebeamforming in the related art is applied.

FIG. 11A is an example of a synthesized image when adaptive beamformingis not applied, and FIG. 11B is an example of a synthesized image whenadaptive beamforming in the related art is applied.

FIG. 12 is a configuration example of an ultrasonic measurementapparatus of an embodiment.

FIG. 13 is a configuration example of an ultrasonic diagnostic apparatuswhich includes the ultrasonic measurement apparatus.

FIGS. 14A to 14C are specification examples of the ultrasonic diagnosticapparatus.

FIGS. 15A to 15C are configuration examples of an ultrasonic transducerelement.

FIG. 16 is a configuration example of an ultrasonic transducer device.

FIGS. 17A and 17B are configuration examples of an ultrasonic transducerelement group each of which is provided in response to each of thechannels.

FIG. 18 is an explanatory diagram of the plane wave propagation regionand a spherical wave propagation region.

FIGS. 19A to 19C are explanatory diagrams of region discriminationprocessing.

FIGS. 20A and 20B are examples of table data.

FIG. 21 is a flow chart illustrating processing of the embodiment.

FIGS. 22A to 22F are examples of ultrasonic images under variousconditions.

FIG. 23 is a characteristics comparison between a signal synthesized bytechnique in the related art and a signal synthesized by technique ofthe embodiment.

FIG. 24 is an example of a plane wave propagation model.

FIGS. 25A and 25B are examples of a spherical wave propagation model.

FIG. 26 is an example of a propagation model indicating propagation ofthe reflected wave (a reception wave).

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, an embodiment will be described. The below-describedembodiment does not unjustly limit the contents of the inventiondisclosed in aspects of the invention. All the configurations describedin the embodiment are not necessarily the essential configurationelement of the invention.

1. Technique of Embodiment

Firstly, technique of the embodiment will be described. Regardingtechnique of performing transmission and reception of a signal in anultrasonic measurement apparatus, there is known technique of focusingon a given measurement point at the time of transmitting an ultrasonicwave signal. For example, with respect to each of the elements in anelement array (for example, corresponding to an ultrasonic transducerdevice described below in FIGS. 16 to 17B) including a plurality ofultrasonic transducer elements 10, a corresponding delay is applied toeach of the elements at the time of driving. Since an ultrasonic wavetransmitted by such technique focuses on a predetermined measurementpoint, if a reflected wave of the ultrasonic wave is received in theelement array, it is possible to acquire an ultrasonic wave signal (in anarrow sense, an ultrasonic image) which focuses on the measurementpoint.

However, in such technique, the measurement point to be focused on isdecided at the time of transmitting the ultrasonic wave signal.Therefore, when it is intended to generate an ultrasonic image in whichfocusing is performed on a plurality of the measurement points (in anarrow sense, all points in an image), there is a need to performdriving of the element array, and transmitting and receiving of theultrasonic wave signal as many times as the number of the plurality ofthe measurement points.

In technique of so-called synthetic aperture technique which is widelyused today, there is no need to perform focusing on a predeterminedmeasurement point at the time of transmission as described above.Specifically, a given transmission wave is transmitted, and a reflectedwave of the transmission wave is received by a plurality of theelements. When a signal processing target point which is a position (maybe multiple, in a narrow sense, points throughout the overall region ofthe image when an ultrasonic image is formed) desired to be focused onis set and it is assumed that transmission waves reach (are propagated)all the set signal processing target points, reception signals which arereceived by the ultrasonic transducer elements include information ofthe reflected waves of all the signal processing target points. Forexample, when it is assumed that there are M signal processing targetpoints r₁ to r_(M) as the signal processing target points, receptionsignals s(t) of given ultrasonic transducer elements are information inwhich all the reflected waves from r₁ to r_(M) are reflected. Herein,the reference sign t is a variable which indicates a time or samplingtiming.

The reception signal s is a function of which the value (an amplitudevalue) varies in a chronological order as described above. However, notall the reflected waves from r₁ to r_(M) are acquired at the sametiming. When it is assumed that a transmission wave is generated fromthe center point of the element array, propagation paths of ultrasonicwaves are different from each other in accordance with the position ofthe signal processing target point, for example, a signal whichcorresponds to r₁ as illustrated in FIG. 1A is received by an elementthrough a transmission and reception path indicated with R₁, and asignal which corresponds to r₂ is received by an element through anothertransmission and reception path indicated with R₂. When the lengths ofthe propagation paths are different from each other, the timing at whichthe signal of the reflected wave is received in the element is differentfrom each other. In other words, in a case of the length of R₁<thelength of R₂ as illustrated in FIG. 1A, a signal corresponding to thesignal processing target point r₁ is presented as a signal of which t isrelatively small (early in time order) in the reception signals s(t) ofthe target element, and a signal corresponding to the signal processingtarget point r₂ is presented as a signal of which t is relativelysignificant (late in time order) in the reception signal s(t), asillustrated in FIG. 1B.

Accordingly, if a time t₁ at which a reflected wave from r₁ is received,and a time t₂ at which a reflected wave from r₂ is received can bespecified, it is considered that a signal s(t₁) includes a signal of thereflected wave from r₁, and a signal s(t₂) includes a signal of thereflected wave from r₂. In this case, there is a relationship of t₁<t₂,as described above.

However, when taking only one element into consideration, in the signalprocessing target points indicated with r₃ and r₄ in FIG. 2, thepropagation paths thereof are R₃ and R₄ being different from each other.However, since there is a relationship of the length of R₃=the length ofR₄, a signal s(t₃) includes both a signal of the reflected wave from r₃and a signal of the reflected wave from r₄. In other words, if thereflected wave is from only one element, it is not easy to separateinformation from only one particular signal processing target point.Originally, if an aperture width is narrow, resolution of a receptionsignal (resolution in an ultrasonic image) is also degraded. Therefore,generally, the element array is formed by arranging the plurality ofelements in the ultrasonic measurement apparatus.

Accordingly, reception signals of the plurality of elements are used inthe synthetic aperture as inevitable processing. Specifically, when theultrasonic measurement apparatus includes first to Nth (N is an integerequal to or greater than 2) ultrasonic transducer elements, a receptionsignal is acquired in each of the elements through transmission of thegiven transmission wave. Therefore, N reception signals s₁(t) tos_(N)(t) can be acquired. Each of the N reception signals includesinformation from a plurality of the signal processing target points (inthe above-referenced assumption, all from r₁ to r_(M)). In this case,since the position of each of the elements is different from each other,R₁₁ to R_(1N) become the paths which respectively correspond to theelements so that a propagation path R₁₁ corresponding to r₁ in a firstelement 10-1 and a propagation path R₁₂ corresponding to r₁ in a secondelement 10-2 are different from each other. Then, values t₁₁ to t_(1N)can be obtained for each element regarding the time when a reflectedwave is received from r₁.

In this case, in order to suitably obtain a reception signal from aparticular signal processing target point, phasing processing may beperformed as illustrated in FIG. 3 so as to align deviation of thereception timing, that is, phase deviation in the reception signal s. InFIG. 3, the extracted signals corresponding to the desired signalprocessing target points among the reception signals s of each of theelements are illustrated in the horizontal axis direction. When thesignal processing target point is an example of r₁, informationcorresponding to t₁₁ to t_(1N) described above may be obtained, ands₁(t₁₁)+s₂(t₁₂) and so on to s_(N)(t_(1N)) may be obtained.

In this case, for example, information corresponding to a differentsignal processing target point r_(m) may be included in s₁(t₁₁) asdescribed above with reference to FIG. 2. However, even when there is arelationship of the length of R_(H)=the length of R_(1m) in a case ofthe first element, if all the elements from the second to Nth areconsidered, it is inconceivable that all of R₁₂ and R_(1m), R₁₃ andR_(1m), and so on to R_(1N) and R_(1m) have uniform relationships (forexample, being equivalent in size). In other words, the times such ast₁₂, t₁₃, and so on to t_(1N) are not information which corresponds tothe signal processing target point r_(m), and the signals such ass₁(t₁₁)+s₂(t₁₂) and so on to s_(N)(t_(1N)) are irrelevant to the signalprocessing target point r_(m). Therefore, when s₁(t₁₁)+s₂(t₁₂) and so onto s_(N)(t_(1N)) are considered in the entirety, it is assumed that theinformation regarding the signal processing target point r_(m) iscancelled so as to be zero (otherwise, a value which can be consideredto be sufficiently close thereto).

Hereinbefore, the N reception signals s are simply added. However,synthesizing processing is not limited thereto. For example, additionmay be performed after each of the N reception signals s is multipliedby a coefficient (a first beamforming coefficient). As theaforementioned first beamforming coefficient, a general apodizationwindow function such as boxcar and hanning may be applied, or anadaptive-type weight which can be obtained through adaptive beamformingusing a MVB method and the like may be applied.

According to the technique described above, there is no need for thegiven measurement point to be focused on at the time of transmission.Therefore, signals of the plurality of elements are synthesized bysuitably performing phasing processing with respect to a receptionsignal, and thus, it is possible to acquire a reception signal whichfocuses on the given signal processing target point. Specifically, whens₁(t₁₁)+s₂(t₁₂) and so on to s_(N)(t_(1N)) are applied as a signal (forexample, a pixel value at a pixel position corresponding to r1 in anultrasonic image) of the signal processing target point r₁, it ispossible to acquire a signal which focuses on a portion of r₁. FIG. 4illustrates an image of synthesis described above. Since FIG. 4 is anexample of a case where one signal processing target point r is set,when it is desired to focus on the plurality of signal processing targetpoints, the synthesizing processing illustrated in FIG. 4 is executed asmany times as the number of the signal processing target points.

However, the above descriptions are given on the basis of the assumptionin that a transmission wave reaches a desired signal processing targetpoint at sufficient intensity, and information corresponding to each ofthe signal processing target points can be acquired as a receptionsignal of each of the elements. However, it is difficult to say such anassumption is realistic. As an example, when performing controlling soas to improve signal intensity and the like by enhancing directivity ofa transmission wave, the transmission wave sometimes reaches only thesignal processing target point within a range for one line (one line ina depth direction in a case where an ultrasonic image is generated). Inthis case, since a reflected wave of a transmission wave is generated inonly the signal processing target point included in the line, eventhough the phasing processing and the synthesizing processing from s₁ tos_(N) are performed, focusing is limited to the signal processing targetpoint which is included in the line. As a result, when it is desired tooutput an ultrasonic image configured to have a number of the lines,there is a need to output transmission waves as many times as the numberof the lines, and thus, the time taken for forming one ultrasonic imageis lengthened.

As a type of technique coping therewith, Patent Literature 1 andNon-Patent Literature 2 disclose technique of transmitting a plane wave.The plane wave is transmitted within a wide range in a transversedirection (a direction along the direction of an element line), and eventhough the depth direction (a direction orthogonal to the direction ofthe element line) is deepened, attenuation in intensity of waves, thatis, sound pressure is insignificant. In other words, since a signal canbe caused to reach a wide range with sufficient intensity even in onetransmission as the plane waves are used, each of the elements can alsoreceive information from a number of the signal processing target pointswith sufficient intensity. Therefore, it is possible to output a signalhaving approximately equivalent resolution with the fewer number oftimes of transmissions compared to a case where the plane waves are notused (for example, in a case where a transmission wave is transmittedwhile being limited to one line as described above).

As illustrated in FIG. 5, when transmission of transmission waves isperformed K times at varied transmission angles, the N reception signalss corresponding to the number of the elements are acquired with respectto one transmission, and a first resolution signal L is acquired bysynthesizing the acquired N reception signals s after phasing processingas illustrated in FIG. 3. This process corresponds to synthesizing inFIG. 4. When focusing on one given signal processing target point, sincethe first resolution signal L is acquired in one transmission, iftransmission is performed K times, it is possible to acquire K firstresolution signals L. Since all the acquired K first resolution signalsL indicate information regarding the one signal processing target pointwhich is currently focused on, it is possible to obtain a secondresolution signal s′ having higher resolution by synthesizing the Kfirst resolution signals L further. FIG. 6 illustrates a flow thereof.The synthesis in FIG. 6 in the transverse direction corresponds to thesynthesis in FIG. 4, that is, the synthesis of the first resolutionsignal L, and the synthesis in FIG. 6 in a vertical directioncorresponds to the synthesis of the second resolution signal s′. SinceFIG. 6 is an example of a case where one signal processing target pointr is set similarly to that in FIG. 4, when it is desired to focus on theplurality of signal processing target points, the synthesizingprocessing illustrated in FIG. 6 is executed as many times as the numberof the signal processing target points.

However, when synthesizing the K first resolution signals Lcorresponding to the number of times of transmissions so as to obtainthe second resolution signal s′, a weight for each of the firstresolution signals L can be variously set. For example, each of thesignals L may be multiplied by a coefficient (a second beamformingcoefficient), and then, addition may be performed. In NonPatent-Literature 1 and Non-Patent Literature 2, the MVB method isapplied as processing for obtaining the second beamforming coefficient.

Hereinbefore, descriptions are given regarding a schematic example ofprocessing technique in the related art using synthetic aperture andplane waves as the transmission waves thereof. However, in the techniquein the related art such as that disclosed in Non-Patent Literature 1, asignal of a wave other than that of the plane wave is not taken intoconsideration. FIG. 7 illustrates a sound field model in the vicinity ofan element array when a plane wave is transmitted from the elementarray. The reference sign DR3 in FIG. 7 indicates a transmissiondirection of the plane wave, and it is shown that the wave surface whichis in the definitely same phase appears to be in a straight line in thedirection of DR3 with respect to the element array and the plane wave ispropagated. Specifically, when straight lines parallel to DR3 are drawnwhile having two end points (the aperture ends) as starting points fromthe element array, it is possible to say that the plane wave ispropagated in the region between the two straight lines. In contrast, asis clear from FIG. 7, the wave surface which is in the same phaseappears to be in a curved line in regions other than that, and it isconsidered that a spherical wave is propagated in the region. In otherwords, even in circumstances where a plane wave is transmitted from theelement array, there exist a region in which the plane wave ispropagated (hereinafter, referred to as the plane wave propagationregion) and a region in which the spherical wave is propagated(hereinafter, referred to as the spherical wave propagation region).

When the sound field in FIG. 7 is assumed, a spherical wave may bepropagated in the signal processing target point in accordance with thetransmission scanning angle or the depth of the signal processing targetpoint (the signal processing target point is positioned in the sphericalwave propagation region). FIGS. 8A and 8B illustrate specificationexamples thereof. For example, in a case of FIG. 8A where the depth isrelatively shallow, if the transmission scanning angle is within a rangefrom −θ_(A) to θ_(A), a plane wave is propagated in the signalprocessing target point. In contrast, in a case of FIG. 8B where thedepth is relatively deep, a range in which a plane wave is propagated inthe signal processing target point becomes the range from −θ_(B) toθ_(B) in which the transmission scanning angle satisfies a relationshipof θ_(B)<θ_(A).

In FIG. 8A, if the transmission scanning angle is caused to be widerthan the range from −θ_(A) to θ_(A), a spherical wave is propagated inthe signal processing target point in a range of θ satisfying arelationship of |θ|>|θ_(A)|, and if the transmission scanning angle iscaused to be equal to or less than the range from −θ_(A) to θ_(A), theplane wave is propagated in the signal processing target point. Fromthis, it is learned that the synthesizing processing for obtaining thesecond resolution signal s′ is more likely to be influenced by a signalother than the plane wave as the transmission scanning angle becomesgreater.

Meanwhile, in the comparison between the examples of FIGS. 8A and 8B, ifthe transmission scanning angle is set to a range from −θ_(c) to θ_(c)satisfying a relationship of θ_(B)<θ_(C)<θ_(A), a plane wave ispropagated in the signal processing target point which is relativelyshallow as described in FIG. 8A. In contrast, in the signal processingtarget point which is relatively deep as described in FIG. 8B, aspherical wave is propagated in the signal processing target point in arange of θ satisfying a relationship of |θ_(B)|<|θ| (≦|θ_(C)|). Fromthis, it is learned that the synthesizing processing for obtaining thesecond resolution signal s′ is more likely to be influenced by a signalother than the plane wave as an observation depth becomes deeper.

When a signal from the spherical wave is used, it is not possible toobtain the effect of the MVB method. The reason will be described withreference to FIGS. 9 to 11B. FIG. 9 is a graph illustrating arelationship between sound pressure of a transmission wave and atransmission angle measured in the signal processing target point whenthe signal processing target point is set to the direction of zerodegrees. Since the signal processing target point is in the direction ofzero degrees, it is also possible to consider FIG. 9 as the graphshowing a relationship between a deviation amount of the transmissionangle with respect to the signal processing target point and the soundpressure (the signal value) of the transmission wave.

As is shown by the sound field model in FIG. 7, the plane wave ispropagated with respect to the given range centering on the direction ofthe transmission angle, and the spherical wave is propagated in rangesother than thereof. Therefore, as illustrated in FIGS. 8A and 8B, theplane wave is propagated with respect to an assumed point in a range atthe given transmission angle centering on a case where a transmissionwave is transmitted in the direction of the signal processing targetpoint (the deviation amount=zero degrees), and the spherical wave ispropagated in a range of which the deviation amount is greater than apredetermined value. As is clear from FIG. 9, since the plane wave andthe spherical wave are different from each other in characteristics ofthe sound pressure, there is a significant difference in the soundpressure when both the plane wave and the spherical wave are sampled. Inother words, when sampling is performed without considering the regionin which the plane wave and the spherical wave are propagated, there aresignificant fluctuations in the sound pressure, as a result thereof.

Therefore, as illustrated in FIG. 10A, among a plurality of the firstresolution signals L (three signals L in a case of the example in FIG.10A) corresponding to the number of times of transmissions of thetransmission wave, the signal L2 which corresponds to the plane wave hasa significant signal value, and in contrast, the signals L1 and L3 whichcorrespond to the spherical wave have extremely small signal values. Thesignal s′ in FIG. 10A is a second resolution signal when an equivalentweight is allocated in all the first resolution signals L as a fixedweight (a weight when the adaptive beamforming such as the MVB method isnot applied). The signal s′ in FIG. 10B is the second resolution signalwhen the weight obtained by the MVB method is used. As is shown by thediagram, since a significant signal is obtained in L2, it is originallyexpected that the signal value of the signal s′ becomes significant inthe signal processing target point. However, the signal value of thesignal s′ in FIG. 10B has rather decreased further than that in FIG.10A.

FIGS. 11A and 11B illustrate luminance images in the vicinity of thesignal processing target point. FIG. 11A shows a luminance image whichis obtained through the processing in FIG. 10A, and FIG. 11B shows aluminance image which is obtained through the processing in FIG. 10B.Originally, it is expected that resolution is improved in FIG. 11Bcompared to that in FIG. 11A, for example, it is expected that theregion having a high luminosity value is concentrated in a narrowerrange. However, as is clear from FIGS. 11A and 11B, there is noimprovement of resolution, and no effect of the MVB method is obtained.

Therefore, this applicant has proposed technique of suitably obtainingan adaptive weight by distinguishing the signal of the plane wave andthe signal other than thereof from each other. Specifically, asillustrated in FIG. 12, the ultrasonic measurement apparatus of theembodiment includes a transmission processing unit 110 which performsprocessing for transmitting ultrasonic waves at a given transmissionangle, a reception processing unit 120 which performs receptionprocessing of an ultrasonic echo with respect to a transmittedultrasonic wave, and a processing unit 130 which performs processingwith respect to a reception signal from the reception processing unit120. The processing unit 130 synthesizes the plurality of firstresolution signals L based on the first beamforming coefficient. Then,the processing unit 130 obtains the second beamforming coefficient forsynthesizing the second resolution signal s′ having high resolutioncompared to the first resolution signal from the plurality of firstresolution signals L based on whether the signal processing target pointbelongs to the plane wave propagation region in which the ultrasonicwave is propagated as a plane wave or belongs to the spherical wavepropagation region in which the ultrasonic wave is propagated as aspherical wave.

Here, the transmission angle controlled by the transmission processingunit 110 denotes a direction in which a plane wave is transmitted, in anarrow sense, and also indicates an angle representing the direction DR3in FIG. 7. As an example, while having a direction perpendicular to theelement array as a standard, as in FIG. 18 described below, an angle αformed by the aforementioned direction and the DR3 may be defined as thetransmission angle. As described above, since it is assumed that thetransmission wave is transmitted multiple times even when the plane waveis used, the aforementioned given transmission angle is not limited tobeing one angle. Therefore, transmission processing may be performedmultiple times at the varied transmission angles.

In this manner, when the plurality of signals (the plurality of firstresolution signals L) having the transmission angles different from eachother are acquired with respect to the given signal processing targetpoint, it is possible to adaptively calculate the second beamformingcoefficient while considering whether each of the signals is a signal ofthe plane wave or a signal of the spherical wave. As an example,processing for selecting only the data of the plane wave from theacquired data may be performed. In this case, it is possible to reducefluctuations in transmission sound pressure and to suitably obtain aneffect of adaptive beamforming processing such as the MVB method.

Hereinafter, a specific example of the system configuration of anultrasonic measurement apparatus 100 of the embodiment will bedescribed, and then, descriptions will be given regarding regiondiscrimination processing for discriminating where the given signalprocessing target point is positioned between the plane wave propagationregion and the spherical wave propagation region. Thereafter, aspecification example of synthesizing processing in which a result ofthe region discrimination processing is applied will be explained.

2. Example of System Configuration

The configuration example of the ultrasonic measurement apparatus 100 ofthe embodiment is illustrated in FIG. 12. FIG. 13 illustrates a specificconfiguration example of an ultrasonic diagnostic apparatus includingthe ultrasonic measurement apparatus of the embodiment. The ultrasonicdiagnostic apparatus may include the ultrasonic measurement apparatus100, an ultrasonic probe 200, and a display unit 300. As illustrated inFIG. 13, the ultrasonic measurement apparatus 100 of the embodiment mayinclude the transmission processing unit 110, the reception processingunit 120, the processing unit 130, a transmission and reception switch140, a digital scan converter (DSC) 150, and a control circuit 160.

The technique of the embodiment is not limited to that applied to theultrasonic measurement apparatus 100 illustrated in FIG. 12. Thetechnique can also be applied to the ultrasonic diagnostic apparatusincluding the ultrasonic measurement apparatus 100 as illustrated inFIG. 13.

The ultrasonic measurement apparatus 100 and the ultrasonic diagnosticapparatus including thereof are not limited to the configurations inFIGS. 12 and 13, and various modifications can be executed by omitting aportion of the configuration elements thereof or adding otherconfiguration elements thereto. In addition, a portion or all of thefunctions of the ultrasonic measurement apparatus 100 of the embodimentand the ultrasonic diagnostic apparatus including thereof may berealized by a server which is connected through communication.

The ultrasonic probe 200 includes the ultrasonic transducer device. Theultrasonic transducer device transmits an ultrasonic beam to a targetobject while scanning the target object along a scanning surface andreceives an ultrasonic echo of the ultrasonic beam. In an example of atype thereof using a piezoelectric element, the ultrasonic transducerdevice includes the plurality of ultrasonic transducer elements (anultrasonic element array), and a substrate in which a plurality of theapertures are arranged in an array shape. An element having a monomorph(unimorph) structure in which a thin piezoelectric element and a metalplate (a vibration film) are pasted together is used as the ultrasonictransducer element. The ultrasonic transducer element (a vibrationelement) converts electrical vibration into mechanical vibration.However, in this case, when the piezoelectric element expands andcontracts within the surface, since the measurements of the pasted metalplate (the vibration film) do not change, there is an occurrence of awarp.

In the ultrasonic transducer device, one channel may be configured toinclude several ultrasonic transducer elements which are arranged to beadjacent to one another, and an ultrasonic beam may be sequentiallymoved while driving a plurality of the channels at a time.

A transducer in a type using the piezoelectric element (a thin filmpiezoelectric element) can be employed as the ultrasonic transducerdevice. However, the embodiment is not limited thereto. For example, atransducer in a type using a capacitive element such as a capacitivemicro-machined ultrasonic transducer (c-MUT) may be employed, or abulk-type transducer may be employed. The ultrasonic transducer elementand the ultrasonic transducer device will be described later further indetail.

The transmission processing unit 110 performs processing fortransmitting an ultrasonic wave to a target object. As illustrated inFIG. 13, the transmission processing unit 110 may include a transmissionpulse generator 111 and a transmission delay circuit 113.

The transmission pulse generator 111 applies a transmission pulsevoltage so as to drive the ultrasonic probe 200. The transmission delaycircuit 113 applies a differential time between the channels regardingthe timing of applying the transmission pulse voltage and decides apropagation direction of ultrasonic waves generated from the pluralityof vibration elements. In this manner, the transmission direction DR3(the transmission angle α) of a plane wave can be controlled by varyinga delay time.

The transmission and reception switch 140 performs switching processingfor transmitting and receiving an ultrasonic wave. The transmission andreception switch 140 protects amplitude pulses during a transmissionfrom being input to a reception circuit, and allows a signal during areception to pass through the reception circuit.

Meanwhile, the reception processing unit 120 performs receptionprocessing of an ultrasonic echo with respect to a transmittedultrasonic wave. As illustrated in FIG. 13, the reception processingunit 120 may include a memory 125. The reception processing unit 120causes the memory 125 to store reception signals (in a narrow sense, s₁to s_(N)) from the ultrasonic probe 200 and outputs the receptionsignals to the processing unit 130. The functions of the memory 125 canbe realized by a memory such as a RAM, or a HDD.

The processing unit 130 performs processing with respect to a receptionsignal from the reception processing unit 120. The functions of theprocessing unit 130 can be realized by hardware such as variousprocessors (CPU and the like) and ASIC (a gate array and the like), or aprogram. As illustrated in FIG. 13, the processing unit 130 includes aregion discrimination processing unit 131, a phasing processing unit132, a first beamforming coefficient calculation unit 133, a firstresolution signal synthesis unit 134, a second beamforming coefficientcalculation unit 135, and a second resolution signal synthesis unit 136.

The region discrimination processing unit 131 performs regiondiscrimination processing for discriminating where a signal processingtarget point which is a processing target, that is, a target point to befocused on is positioned in any one of the plane wave propagation regionand the spherical wave propagation region. The region discriminationprocessing will be described later in detail.

The phasing processing unit 132 performs phasing processing asillustrated in FIG. 3. The phasing processing unit 132 may be applied soas to perform widely known phasing processing. However, the phasingprocessing unit 132 of the embodiment may perform the phasing processingbased on a result of discriminating whether the signal processing targetpoint exists in the plane wave propagation region or in the sphericalwave propagation region. The processing of the phasing processing unit132 performed based on the result of the region discriminationprocessing will be described later with reference to FIG. 24 and thelike, as a modification example.

The first beamforming coefficient calculation unit 133 calculates thefirst beamforming coefficient which is a coefficient used whensynthesizing the reception signals s₁ to s_(N) after the phasingprocessing. In the embodiment, all the coefficients may be set to 1 asdescribed above, or a fixed value set in advance may be used. In such acase, since there is no need to perform processing for adaptivelyobtaining the first beamforming coefficient, the first beamformingcoefficient calculation unit 133 may be omitted.

The first resolution signal synthesis unit 134 performs the synthesizingprocessing of reception signals of the N elements with respect to onetransmission wave based on the reception signals s₁ to s_(N) after thephasing processing, and the first beamforming coefficient. Specifically,the first resolution signal L may be obtained by performing theabove-described processing with reference to FIG. 4.

The second beamforming coefficient calculation unit 135 calculates thesecond beamforming coefficient which is a coefficient used whensynthesizing the second resolution signal s′ based on the firstresolution signal L as many as the number (K) of times of transmissions.As described above, in the embodiment, the result of the regiondiscrimination processing is used when calculating the secondbeamforming coefficient. Detailed descriptions will be given later.

The second resolution signal synthesis unit 136 obtains a signalregarding the given signal processing target point by using K times oftransmissions of transmission waves and information of the N elementsbased on the K first resolution signals L obtained by the firstresolution signal synthesis unit 134, and the second beamformingcoefficient. Specifically, the second resolution signal s′ may beobtained by performing the above-described processing with reference toFIG. 6.

The DSC 150 performs scanning conversion processing with respect toB-mode image data. For example, the DSC 150 converts a line signal intoan image signal through interpolation processing such as a bilinearmethod. The control circuit 160 is mutually connected to each of theunits of the ultrasonic measurement apparatus 100 and controls each ofthe connected units.

The display unit 300 displays image data for displaying generated in theDSC 150 by using the second resolution signal s′. For example, thedisplay unit 300 may be realized by a liquid crystal display, an organicEL display, an electronic paper, or the like.

Here, FIGS. 14A to 14C illustrate specific examples of configurations ofthe instruments in the ultrasonic diagnostic apparatus (in a broadsense, an electronic instrument) of the embodiment. FIG. 14A is anexample of a portable ultrasonic diagnostic apparatus, and FIG. 14B isan example of a stationary ultrasonic diagnostic apparatus. FIG. 14C isan example of an integrated ultrasonic diagnostic apparatus equippedwith the built-in ultrasonic probe 200.

The ultrasonic diagnostic apparatus in FIGS. 14A and 14B includes theultrasonic probe 200 and an ultrasonic measurement apparatus main body101 (in a broad sense, an electronic instrument main body). Theultrasonic probe 200 and the ultrasonic measurement apparatus main body101 are connected to each other by a cable 210. A probe head 220 isprovided at the tip end portion of the ultrasonic probe 200, and thedisplay unit 300 for displaying an image is provided in the ultrasonicmeasurement apparatus main body 101. In FIG. 14C, the ultrasonic probe200 is built in the ultrasonic measurement apparatus 100 having thedisplay unit 300. In a case of FIG. 14C, the ultrasonic measurementapparatus 100 can be realized by a general-purpose portable informationterminal, for example, a smartphone.

FIGS. 15A to 15C illustrate a configuration example of an ultrasonictransducer element 10 of the ultrasonic transducer device. Theultrasonic transducer element 10 includes a vibration film (a membraneand a support member) 50 and a piezoelectric element portion. Thepiezoelectric element portion includes a first electrode layer (a lowerelectrode) 21, a piezoelectric layer (a piezoelectric film) 30, and asecond electrode layer (an upper electrode) 22.

FIG. 15A is a plan view of the ultrasonic transducer element 10 which isformed in a substrate (a silicon substrate) 60 seen in a directionvertical to the substrate 60 on the element forming surface side. FIG.15B is a cross-sectional view illustrating a cross section taken alongline A-A′ in FIG. 15A. FIG. 15C is a cross-sectional view illustrating across section taken along line B-B′ in FIG. 15A.

The first electrode layer 21 is formed with a metallic thin film, forexample, on an upper layer of the vibration film 50. The first electrodelayer 21 may be a wire which extends to the outside of an elementforming region as illustrated in FIG. 15A and is connected to theadjacent ultrasonic transducer element 10.

For example, the piezoelectric layer 30 is formed with a lead zirconatetitanate (PZT) thin film and is provided so as to cover at least aportion of the first electrode layer 21. The material of thepiezoelectric layer 30 is not limited to PZT. For example, lead titanate(PbTiO3), lead zirconate (PbZrO3), titanate lead lanthanum ((Pb, La)TiO3), and the like may be used.

For example, the second electrode layer 22 is formed with a metallicthin film and is provided so as to cover at least a portion of thepiezoelectric layer 30. The second electrode layer 22 may be a wirewhich extends to the outside of an element forming region as illustratedin FIG. 15A and is connected to the adjacent ultrasonic transducerelement 10.

For example, the vibration film (the membrane) 50 is provided so as toblock an aperture 40 with a two-layer structure of a SiO2 thin film anda ZrO2 thin film. The vibration film 50 supports the piezoelectric layer30, and first and second electrode layers 21 and 22. The vibration film50 vibrates in accordance with expansion and contraction of thepiezoelectric layer 30 and can generate ultrasonic waves.

The aperture 40 is formed by performing etching such as reactive ionetching (RIE) from a rear surface (a surface with no element formedthereon) side of the substrate 60 (the silicon substrate). The resonancefrequency of the ultrasonic wave is decided in accordance with the sizeof an aperture portion 45 of the aperture 40, and the ultrasonic wave isemitted to the piezoelectric layer 30 side (in the front direction fromthe back on the sheet surface in FIG. 15A).

The lower electrode (a first electrode) of the ultrasonic transducerelement 10 is formed by the first electrode layer 21, and the upperelectrode (a second electrode) is formed by the second electrode layer22. Specifically, a portion of the first electrode layer 21 covered withthe piezoelectric layer 30 forms the lower electrode, and a portion ofthe second electrode layer 22 covering the piezoelectric layer 30 formsthe upper electrode. In other words, the piezoelectric layer 30 isprovided so as to be interposed between the lower electrode and theupper electrode.

FIG. 16 illustrates a configuration example of the ultrasonic transducerdevice (an element chip). The ultrasonic transducer device in thisconfiguration example includes a plurality of ultrasonic transducerelement groups UG1 to UG64, drive electrode lines DL1 to DL64 (in abroad sense, first to nth drive electrode lines. The factor n is aninteger equal to or greater than 2), and common electrode lines CL1 toCL8 (in a broad sense, the first to mth common electrode lines. Thefactor m is an integer equal to or greater than 2). The number (n) ofthe drive electrode lines and the number (m) of the common electrodelines are not limited to the numbers illustrated in FIG. 16.

The plurality of ultrasonic transducer element groups UG1 to UG64 arearranged in 64 columns along a second direction D2 (a scan direction).Each of the ultrasonic transducer element groups UG1 to UG64 has theplurality of ultrasonic transducer elements which are arranged along afirst direction D1 (a slice direction).

FIG. 17A illustrates an example of the ultrasonic transducer elementgroup UG (UG1 to UG64). In FIG. 17A, the ultrasonic transducer elementgroup UG is configured to have first to fourth element columns. Thefirst element column is configured to have ultrasonic transducerelements UE11 to UE18 which are arranged along the first direction D1,and the second element column is configured to have ultrasonictransducer elements UE21 to UE28 which are arranged along the firstdirection D1. The third element column (UE31 to UE38) and the fourthelement column (UE41 to UE48) are similar thereto as well. The driveelectrode lines DL (DL1 to DL64) are commonly connected to the first tofourth element columns, and the common electrode lines CL1 to CL8 areconnected to the ultrasonic transducer elements of the first to fourthelement columns.

The ultrasonic transducer element group UG in FIG. 17A is configured tobe one channel of the ultrasonic transducer device. In other words, thedrive electrode line DL corresponds to the drive electrode line in onechannel and a transmission signal of one channel from a transmissioncircuit is input to the drive electrode line DL. A reception signal ofone channel from the drive electrode line DL is output from the driveelectrode line DL. The number of element columns configuring one channelis not limited to four as described in FIG. 17A. The column may be fewerthan four columns or more than four columns. For example, as illustratedin FIG. 17B, the number of the element columns may be one.

As illustrated in FIG. 16, the drive electrode lines DL1 to DL64 (thefirst to nth drive electrode lines) are wired along the first directionD1. A jth (j is an integer of 1≦j≦n) drive electrode line DLj (a jthchannel) among the drive electrode lines DL1 to DL64 is connected to thefirst electrode (for example, the lower electrode) included in theultrasonic transducer element of a jth ultrasonic transducer elementgroup UGj.

During a transmission period in which ultrasonic waves are emitted,transmission signals VT1 to VT64 are supplied to the ultrasonictransducer element via the drive electrode lines DL1 to DL64. During areception period in which ultrasonic echo signals are received,reception signals VR1 to VR64 are output from the ultrasonic transducerelement to the drive electrode lines DL1 to DL64.

The common electrode lines CL1 to CL8 (the first to mth common electrodelines) are wired along the second direction D2. The second electrodeincluded in the ultrasonic transducer element is connected to any oneamong the common electrode lines CL1 to CL8. Specifically, for example,as illustrated in FIG. 16, an ith (i is an integer of 1≦i≦m) commonelectrode line CLi among the common electrode lines CL1 to CL8 isconnected to the second electrode (for example, the upper electrode)included in the ultrasonic transducer element which is arranged in anith row.

A common voltage VCOM is supplied to the common electrode lines CL1 toCL8. The common voltage VCOM may be a constant direct current voltageand does not need to be 0V, that is, ground potential.

During the transmission period, a differential voltage between thetransmission signal voltage and the common voltage is applied to theultrasonic transducer element, and an ultrasonic wave at a predeterminedfrequency is emitted.

The arrangement of the ultrasonic transducer elements is not limited tothe matrix arrangement illustrated in FIG. 16 and may be a so-calledzig-zag arrangement or the like.

FIGS. 17A to 17B illustrate cases where one ultrasonic transducerelement serves as both a transmission element and a reception element.However, the embodiment is not limited thereto. For example, theultrasonic transducer element for a transmission element and theultrasonic transducer element for a reception element may be separatelyprovided in an array shape.

3. Region Discrimination Processing

Subsequently, descriptions will be given regarding the regiondiscrimination processing which is performed by the regiondiscrimination processing unit 131 of the processing unit 130. Asillustrated in FIG. 18, when a given transmission angle α is decided, aplane wave is transmitted to a region which is decided based on thetransmission angle α and the width (the aperture width) of theultrasonic transducer element, and the region becomes the plane wavepropagation region. The regions outside the plane wave propagationregion become the spherical wave propagation region.

In other words, the plane wave propagation region and the spherical wavepropagation region are the regions which vary in accordance with thetransmission angle of an ultrasonic wave of the transmission processingunit 110. In other words, when the transmission angle α is decided, theplane wave propagation region and the spherical wave propagation regionfor a can be decided.

Therefore, as illustrated in FIG. 5 and the like, when K values fromfirst to Kth transmission angles can be taken for the transmissionangle, it is possible to consider the plane wave propagation regions andthe spherical wave propagation regions in the number of K. For example,the first plane wave propagation region and the first spherical wavepropagation region corresponding to the first transmission angle aredecided, and the second plane wave propagation region and the secondspherical wave propagation region corresponding to the secondtransmission angle are decided.

However, processing contents cannot be fixed with only the plane wavepropagation region and the spherical wave propagation region so that theposition of the signal processing target point which is the processingtarget also needs to be considered. For example, it is assumed that thegiven transmission angle is decided and the plane wave propagationregion and the spherical wave propagation region are decided asillustrated in FIG. 18. Also in such a case, if the signal processingtarget point is at the position indicated by A1 in FIG. 18, the signalprocessing target point is in the plane wave propagation region.However, if the signal processing target point is at the positionindicated by A2, the signal processing target point is in the sphericalwave propagation region.

In other words, the processing unit 130 needs to acquire the position ofthe signal processing target point which is a target desired to befocused on. Thereafter, the processing unit 130 performs the regiondiscrimination processing regarding where the signal processing targetpoint exists in any one of the plane wave propagation region and thespherical wave propagation region.

Specifically, the transmission processing unit 110 may performprocessing for transmitting the first to Kth (K is an integer equal toor greater than 2) ultrasonic waves at the first to Kth transmissionangles, and the processing unit 130 may perform the regiondiscrimination processing for discriminating where the signal processingtarget point exists in any one of the ith plane wave propagation regionand the ith spherical wave propagation region corresponding to the ithultrasonic wave, based on the ith (i is an integer of 1≦i≦K)transmission angle and the position of the signal processing targetpoint.

In this manner, when one given point is decided for the signalprocessing target point, the region discrimination processing isperformed K times as many as the number of the transmission angles (thenumber of times of transmissions), thereby acquiring K results of theregion discrimination processing. Here, the result of the regiondiscrimination processing denotes information which indicates whetherthe signal processing target point exists in the plane wave propagationregion or exists in the spherical wave propagation region. For example,the information may consist of two values so as to exhibit 1 whenexisting in the plane wave propagation region and to exhibit 0 whenexisting in the spherical wave propagation region.

As described above, since the signal processing target point isgenerally set in multiple numbers, as many the results of the regiondiscrimination processing as M×K which is multiplication of the settingnumber M of the signal processing target points and the number K of thetransmission angles are actually acquired.

Various types of specific technique can be considered for the regiondiscrimination processing when the position of the signal processingtarget point and the transmission angle are decided. As an example, theprocessing unit 130 may perform the region discrimination processingbased on a first direction DR1 in which a first aperture end leads tothe signal processing target point r among the aperture ends at whichthe plurality of ultrasonic transducers are provided, a second directionDR2 in which a second aperture end that is different from the firstaperture end leads to the signal processing target point r among theapertures, and the transmission angle α (in a broad sense, thetransmission direction DR3) of an ultrasonic wave. The first apertureend may be a position of a first ultrasonic transducer corresponding tothe aperture end among the first to Nth ultrasonic transducers, and thesecond aperture end may be a position of the Nth ultrasonic transducer.

As described above with reference to FIGS. 7 and 18, a boundary betweenthe plane wave propagation region and the spherical wave propagationregion is formed by two straight lines L1 and L2 passing through theaperture ends in the same direction as the transmission direction DR3 ofthe plane wave. In other words, as illustrated in FIG. 19A, the positionof the signal processing target point on the line L1 corresponds to thelimit line on one side when the signal processing target point isincluded in the plane wave propagation region, and as illustrated inFIG. 19B, the position of the signal processing target point on the lineL2 corresponds to the limit line on the other side when the signalprocessing target point is included in the plane wave propagationregion.

In FIG. 19A, since the line L1 is a straight line in the direction ofDR3 (parallel to DR3), a case where the signal processing target pointis on the line L1 denotes that the first direction DR1 in which thefirst ultrasonic transducer is connected to the signal processing targetpoint r and the transmission direction DR3 of an ultrasonic wavecoincide with each other. Similarly, as illustrated in FIG. 19B, a casewhere the signal processing target point is on the line L2 denotes thatthe second direction DR2 in which the Nth ultrasonic transducer isconnected to the signal processing target point r and the transmissiondirection DR3 of an ultrasonic wave coincide with each other. In otherwords, the region discrimination processing can be performed bydetermining a relationship between DR1 and DR3, and a relationshipbetween DR2 and DR3.

Various types of technique can be considered for comparison processingof directions as well. For example, the comparison processing may beperformed regarding the degree of an angle with respect to a givenstandard direction. When a direction perpendicular to the element array(a depth direction z) is set as the standard direction, as illustratedin FIG. 18, it is possible to define the angle θ1 corresponding to DR1,the angle θ2 corresponding to DR2, and the angle α corresponding to DR3.

In the circumstances as illustrated in FIG. 19A, there is a relationshipof θ1=α, and in the circumstances as illustrated in FIG. 19B, there is arelationship of θ2=α. Then, as illustrated in FIG. 19C, in thecircumstances where the signal processing target point is in the planewave propagation region, α becomes a value between θ1 and θ2. In anexample having a relationship of θ1>θ2 as that in the example of FIG.19C, there exists a relationship of θ2<α<θ1. In other words, in theexamples of FIGS. 19A to 19C, it may be determined that the signalprocessing target point is in the plane wave propagation region when therelationship of θ2≦α≦θ1 is satisfied, and it may be determined that thesignal processing target point is in the spherical wave propagationregion when there is a relationship of α<θ2 or θ1<α.

Even though specific determination expression varies in accordance witha method of setting the standard direction or the condition of arelationship of magnitude between θ1 and θ2, it is possible to performthe region discrimination processing based on a relationship among DR1,DR2, and DR3 as described above.

In the embodiment, the region discrimination processing may be performedfor each round of the processing timing by using the above-describedexpressions and the like. However, the embodiment is not limitedthereto. For example, there are one or multiple types of the ultrasonicprobe 200, and it is assumed that the type is possibly specified to bein a small number. In other words, since it is possible to know theconfiguration of the ultrasonic transducer element array in advance, itis also possible to know the positions of the aperture ends. Inaddition, even though various modifications can be executed regardingthe degree of the range of the transmission angle and the degree of thevariable width of the angle used in the scanning, the scanning issupposed to be limited to a certain pattern. Therefore, it is alsopossible to know the transmission angle α in advance. When the size andthe like of an ultrasonic image to be acquired are taken intoconsideration, it is also possible to specify the setting (the number ofthe signal processing target points, the position, and the like) of thesignal processing target point desired to be focused on in advance.

As described above, as the signal processing target point, the angle,and the positions of the aperture ends are decided, the regiondiscrimination processing is ready to be performed. Therefore, it ispossible to acquire all thereof in advance. In this case, thedetermination may be performed in advance and only the result thereofmay be retained as the table data, without acquiring the result byperforming the determination using the above-described expressions andthe like for each time.

In other words, the ultrasonic measurement apparatus 100 mayadditionally include a storage unit (not illustrated in FIG. 12 and thelike) which stores the table data in which information indicating wherethe signal processing target point exists in any one of the plane wavepropagation region and the spherical wave propagation region correspondsto the given signal processing target point in each transmission angleamong the plurality of transmission angles of an ultrasonic wave fromthe transmission processing unit 110. The processing unit 130 mayperform the region discrimination processing based on the table data.

In this manner, there is no need to perform specific computation and thelike every time, and the region discrimination processing can berealized with reference to the table data. Therefore, it is possible toreduce a burden to the processing and to speed up the regiondiscrimination processing.

FIGS. 20A and 20B illustrate examples of the table data. As describedabove, in order to decide the existence in any one of the plane wavepropagation region and the spherical wave propagation region, the signalprocessing target point r and the transmission angle α need to bedecided. In FIG. 20A, any one between the information indicating theexistence in the plane wave propagation region (in FIG. 20A, data of“1”) and the information indicating the existence in the spherical wavepropagation region (in FIG. 20A, data of “0”) corresponds to each of theK transmission angles, with respect to the one given signal processingtarget point r_(P). Accordingly, when the plurality (M) of signalprocessing target points are set, M items of the table data are retainedas illustrated in FIG. 20A, or the table data having M×K items ofinformation is used as illustrated in FIG. 20B.

Furthermore, when the ultrasonic probe 200 is replaceable and aplurality of the ultrasonic probes 200 having the aperture widthsdifferent from one another are possibly connected thereto, the result ofthe region discrimination processing varies in accordance with theaperture width. Therefore, there is a need to retain the table datacorresponding to each aperture width. For example, the table dataillustrated in FIG. 20B may be retained as many as the assumed types ofthe ultrasonic probe 200.

Otherwise, only a portion of the table data, for example, the table dataassumed to be frequently used such as the aperture width, the signalprocessing target point, and the transmission angle may be retainedinstead of retaining all the table data. In this case, the regiondiscrimination processing may be performed by using the table data incircumstances where the aperture width, the signal processing targetpoint, and the transmission angle corresponding to the table data areused, and in other circumstances, the region discrimination processingmay be performed every time by using the above-described determinationexpressions and the like. Otherwise, when the region discriminationprocessing is performed by using the above-described determinationexpressions and the like, the result of the discrimination processingmay be retained as the table data. In this case, for example, whenprocessing using the same aperture width, signal processing targetpoint, and transmission angle is performed twice or more, the regiondiscrimination processing is performed by using the determinationexpressions and the like in the first processing and the result thereofis retained as the table data, and the region discrimination processingis performed by using the table data after the second time.

In the above-described region discrimination processing, descriptionsare given regarding the technique which is performed based on DR1 toDR3. However, the processing is not limited thereto. As described abovewith reference to FIGS. 9 to 11B, an occurrence of a problem in thetechnique in the related art is caused by the fact in that fluctuationsin the sound pressure increase when the spherical wave is used in theprocessing. In other words, even though it is determined to be the planewave propagation region by the above-described discrimination technique,if there are fluctuations in the sound pressure, a problem similar tothat in the technique in the related art may occur.

Particularly, as is shown by FIG. 9, since the sound pressure originallytends to decrease in the vicinity of the boundary between the plane wavepropagation region and the spherical wave propagation region, a problempossibly occurs regarding accuracy of the processing due to an errorcaused by mixed noise, and the like. Therefore, in the embodiment, amargin may be applied to the region discrimination processing so as toprevent the fluctuations further. As an example, instead of the rangeindicated by D1 in FIG. 9 so as to be the plane wave propagation region,D2 which is the narrower angle range may be set as the plane wavepropagation region. In this manner, the fluctuations in the soundpressure can be prevented, and thus, it is possible to perform thefavorably accurate synthetic aperture processing.

4. Details of Processing

The descriptions above are the region discrimination processingperformed by the region discrimination processing unit 131. In thephasing processing unit 132 of the processing unit 130, a phasedifference (the delay time) in each element is obtained as illustratedin FIG. 3, and phasing processing for reducing deviation of the phasedifference (in a narrow sense, make the phase difference be zero) isperformed. When obtaining the delay time, propagation models of atransmission wave and a reflected wave (a reception wave) are taken intoconsideration. However, the plane wave propagation model and thespherical wave propagation model are different from each other.Accordingly, it is possibly considered that there is a need to causephasing processing to vary depending on whether the signal processingtarget point exists in the plane wave propagation region or exists inthe spherical wave propagation region.

However, in the embodiment, as described below, signals of the planewave are used in calculation of the second beamforming coefficient or inthe synthesizing processing of the second resolution signal s′, butsignals of the spherical wave are not used. In other words, even thoughprocessing is performed while taking the spherical wave propagationmodel into consideration in the phasing processing, the result thereofis not used in the successive processing. Therefore, there is lownecessity of using the result of the region discrimination processing inthe phasing processing. In consideration of the aforementionedcircumstance, general (not using the result of the region discriminationprocessing) phasing processing is performed in the embodiment. Since thegeneral phasing processing is widely known, detailed descriptionsthereof will be omitted.

when the reception processing unit 120 performs the reception processingin the first to Nth ultrasonic transducers for an ultrasonic echo withrespect to a transmitted ultrasonic wave, the processing unit 130performs the phasing processing with respect to the first to Nthreception signals corresponding to the first to Nth ultrasonictransducers as described above. Thereafter, the processing unit 130synthesizes the first to Nth reception signals which have been subjectedto the phasing processing, based on the first beamforming coefficient,thereby generating the first resolution signal L. The processingcorresponds to the synthesizing processing in the transverse directionof FIG. 6 and may be performed by the first resolution signal synthesisunit 134.

The processing unit 130 synthesizes the first to Nth synthetic signalsof the first resolution, and the signal processing target point is setto be focused on, thereby generating an output signal of the secondresolution having higher resolution compared to the first resolution.The processing is configured to include calculation processing of thesecond beamforming coefficient performed by the second beamformingcoefficient calculation unit 135, and the synthesizing processingperformed by the second resolution signal synthesis unit 136.

Specifically, the processing unit 130 (the second beamformingcoefficient calculation unit 135) selects a coefficient computationfirst resolution signal from the plurality of first resolution signals Lbased on the result of the region discrimination processing regardingwhether the signal processing target point belongs to the plane wavepropagation region or belongs to the spherical wave propagation region,thereby obtaining the second beamforming coefficient based on theselected coefficient computation first resolution signal.

Herein, the coefficient computation first resolution signal is a signalwhich contributes to improvement of resolution when being used incalculation of the second beamforming coefficient, specifically oncondition of small fluctuations in the sound pressure of thetransmission wave used when acquiring the signal value. As illustratedin FIG. 9, in order to reduce the fluctuations in the sound pressure ofthe transmission wave, it is acceptable as long as the transmission waveis in the plane wave, that is, the signal processing target point as theprocessing target is in the plane wave propagation region. In otherwords, it is acceptable as long as the first resolution signal of whichthe signal processing target point is in the plane wave propagationregion is selected as the coefficient computation first resolutionsignal from the plurality of first resolution signals L.

Since fluctuations in the sound pressure of a transmission wave can beprevented by using the coefficient computation first resolution signal,it is possible to suitably calculate the second beamforming coefficient.When the first resolution signal to be the processing target is decided,there are various types of known technique as the technique ofadaptively obtaining a beamforming coefficient from a signal of theprocessing target, such as the MVB method (the Capon method) and thelinear prediction method. Since the aforementioned technique can bewidely applied to the embodiment, specified descriptions for thetechnique of the coefficient computation will be omitted.

Thereafter, the processing unit 130 (the second resolution signalsynthesis unit 136) synthesizes the selected coefficient computationfirst resolution signal based on the obtained second beamformingcoefficient, thereby generating the second resolution signal.

In this case, the first resolution signal corresponding to the sphericalwave is not used in calculation of the second beamforming coefficientand is not used in the synthesizing processing of the second resolutionsignal s′ either. In this regard, it is possible to consider that 0 isallocated as the second beamforming coefficient with respect to thefirst resolution signal L corresponding to the spherical wave. In thismanner, it is possible to prevent the processing from being performed asdescribed in FIGS. 10B and 11B by setting a signal caused by the planewave as the processing target in the processing for obtaining the secondresolution signal s′, that is, the calculation processing of the secondbeamforming coefficient and the synthesizing processing using the secondbeamforming coefficient. Therefore, it is possible to improve resolutionof the signal (the output image) which is obtained through thesynthesizing processing. In other words, even though the transmissionscanning angle is increased or the depth of the signal processing targetpoint is deepened, it is possible to obtain the effect of the adaptivebeamforming in the MVB method and the like.

The aforementioned flow will be described with reference to expressions.The following Expression (1) represents the synthesizing processingperformed after the phasing processing and the phasing processingdescribed above. In the following Expression (1), the factor s′ (r_(p))is a signal value (an output signal) in a signal processing target pointr_(p), and the factor r_(p) is a vector indicating the position of thesignal processing target point. The factor K is the total number oftimes of transmissions of a transmission wave. The factor N is thenumber of the elements. The factor k is a transmission number. Thefactor n indicates a reception element number. The factor a_(n) is awindow function in apodization, and specifically, is the above-describedfirst beamforming coefficient. Specifically, the factor a_(k) is theabove-described second beamforming coefficient and can be obtained froma signal of a plane wave by using the result of the regiondiscrimination processing as described above. The factor s_(k,n)indicates a reception signal in the nth element corresponding to the kthtransmission wave. The above-described signals s₁ to s_(N) correspond tos_(k,n) when n=1 to N in a case where the transmission wave is specifiedas one given wave.

$\begin{matrix}{{s^{\prime}\left( r_{p} \right)} = {\sum\limits_{k = 1}^{K}{\sum\limits_{n = 1}^{N}{a_{k}a_{n}{s_{k,n}\left( {t_{ToF}\left( {r_{p},k,n} \right)} \right)}}}}} & (1)\end{matrix}$

In the above Expression 1, the factor t_(ToF) (r_(p), k, n) is afunction for calculating a propagation time (the delay time) and isactually corresponds to the region discrimination processing and thecalculation processing for the propagation time performed based on theresult of the region discrimination processing. The output of thefunction becomes the time (the sampling timing and the sampling number)of a signal corresponding to the signal processing target point r_(p)which is obtained through the kth transmission among reception signalss_(k,n) of the element n.

The signal s_(k,n) (t_(ToF)) is a function for executing the phasingprocessing, and a signal value of a desired sampling number is sampledfrom the reception signals s_(k,n).

When the factor L(r_(p)) is defined through the following Expression 2,L(r_(p)) corresponds to reception focus processing.

$\begin{matrix}{{L\left( r_{p} \right)} = {\sum\limits_{n = 1}^{N}{a_{n}s_{k,n}}}} & (2)\end{matrix}$

Specifically, the above-described expression is the processing forsynthesizing the first resolution signal which is focused on the signalprocessing target point represented by r_(p). Since when the signalprocessing target point r_(p) is changed, the first resolution signal isobtained throughout the overall region of the observation region, andimaging is performed, only a reception focus is obtained and an image ofwhich the transmission focus cannot be obtained is obtained, the firstresolution signal is a low-resolution signal having resolution lowerthan that of the below-described second resolution signal. It is becausesince the transmission angle is one given angle in a stage whereL(r_(p)) is obtained, it is not possible to perform the processing forselecting a transmission wave which is in focus at the time oftransmission with respect to a particular signal processing target pointamong the plurality of transmission angles. In other words, it is notpossible to obtain a transmission focus in regard to the point wheresignals of the plurality of transmission angles are not synthesized. Thefirst beamforming coefficient a_(n) is set as 1 in all the examplesthroughout the descriptions given with reference to FIG. 4 and the like.However, a general apodization window function such as boxcar andhanning may be applied, or the adaptive-type weight which can beobtained through the adaptive beamforming may be applied.

Meanwhile, when L(r_(p)) is defined as above, the above Expression 1 canbe modified as the following Expression 3.

$\begin{matrix}{{s^{\prime}\left( r_{p} \right)} = {\sum\limits_{k = 1}^{K}{a_{k}{L\left( r_{p} \right)}}}} & (3)\end{matrix}$

As is shown by the above Expression 3, the factor s′(r_(p)) is a signalwhich is obtained by synthesizing the first resolution signal using thesecond beamforming coefficient and is the second resolution signal whichis in focus for transmission and reception with respect to the signalprocessing target point r_(p). Since when the signal processing targetpoint r_(p) is changed, the second resolution signal is obtainedthroughout the overall region of the observation region, and imaging isperformed, a reception focus and a transmission focus can be obtainedthroughout the overall region of an image, the second resolution signalis a high-resolution signal having higher resolution compared to thefirst resolution signal. It is because the signals of the plurality oftransmission angles are synthesized for s′(r_(p)) being different fromL(r_(p)). The second beamforming coefficient a_(k) can be adaptivelyobtained by using the coefficient computation first resolution signal Las described above.

FIG. 21 illustrates a flow chart describing the processing of theembodiment. When the processing starts, first, the reception processingof a signal which is reflected from a test object is performed by thereception processing unit 120 (S101). Then, phasing addition processingis performed in a plurality of the lines, thereby obtaining the firstresolution signal L (S102). The processing in S102 can be realizedthrough the processing for obtaining the delay time with respect to eachof the elements, and the processing for synthesizing the receptionsignal s of each of the elements by using the obtained delay time.

Thereafter, the region discrimination processing is performed, and basedon the result of the processing thereof, the first resolution signal Lused in the adaptive beamforming processing is selected (S103). Thesignal which is selected in S103 corresponds to the above-describedcoefficient computation first resolution signal.

The second beamforming coefficient is calculated by using the selectedfirst resolution signal L (S104), and the calculated second beamformingcoefficient is used to synthesize the first resolution signal L, therebyobtaining the second resolution signal s′ (S105). The synthesis targetin S105 may be the coefficient computation first resolution signal amongthe first resolution signals L as described above.

FIGS. 22A to 22F illustrate comparison of the ultrasonic images (B-modeimages) acquired by the technique in the related art and the techniqueof the embodiment. FIGS. 22A to 22F are images which are acquired byusing computer simulation. FIGS. 22A to 22C are ultrasonic images of thesignal processing target point of which the depth is relatively shallow,and FIGS. 22D to 22F are ultrasonic images of the signal processingtarget point of which the depth is relatively deep. FIGS. 22A and 22Dare ultrasonic images when the MVB method is not applied, FIGS. 22B and22E are ultrasonic images when the MVB method performed by the techniquein the related art is applied, FIGS. 22C and 22F are ultrasonic imageswhen the MVB method performed by the technique of the embodiment isapplied.

As is shown by the comparison between FIGS. 22A and 22B, when the depthis shallow, resolution is improved even though the MVB method isperformed by the technique in the related art. It is because arrivalprobability of the plane wave is high when the depth is shallow asdescribed above with reference to FIG. 8A, and particularly, because thetransmission scanning angle set in the simulation is within the rangefrom −θ_(A) to θ_(A) (or the range close to thereof) mentioned in FIG.8A. In this case, the technique in the related art is sufficient toperform the processing. However, as is shown by FIG. 22C, resolution isnaturally improved by the technique of the embodiment as well.

As is shown by the comparison between FIGS. 22D and 22E, there is noimprovement of resolution in the MVB method performed by the techniquein the related art when the depth is deep. It is because the conditionon the transmission scanning angle becomes strict when the depth isdeep, as described above with reference to FIG. 8B so that thetransmission scanning angle set in the simulation is not included in therange from −θ_(B) to θ_(B) mentioned in FIG. 8B. In contrast, as isshown by FIG. 22F, resolution is improved by the technique of theembodiment, thereby being ascertained to be excellent compared to thetechnique in the related art.

FIG. 23 illustrates a relationship of signal intensity with respect tovariations of the azimuthal direction. FIG. 23 illustrates informationfor the signal processing target point of which the depth is relativelydeep while being corresponding to those in FIGS. 22D to 22F. As is shownby FIG. 23, the difference in the signal values in the vicinity of thepeak is not significant between the case not applied with the MVB methodand the case applied with the MVB method performed by the technique inthe related art, thereby being ascertained that resolution is notimproved. In contrast, when being applied with the MVB method based onthe result of the region discrimination processing as that of theembodiment, it is clear that the peak becomes rapid compared to the casenot applied with the MVB method, thereby being ascertained thatresolution is improved.

5. Modification Example

In the above-described embodiment, signals of the plane wave (thecoefficient computation first resolution signal) are used in calculationof the second beamforming coefficient and synthesizing processing of thesecond resolution signal s′, and signals of the spherical wave are notused therein. However, the technique of the embodiment is not limitedthereto. The signals of the spherical wave may also be utilized whenobtaining the second resolution signal s′. However, as illustrated inFIG. 9, since the plane wave and the spherical wave are significantlydifferent from each other in the sound pressure, if the sound pressureis treated in an equivalent manner, a problem illustrated in FIGS. 10Band 11B occurs. Therefore, as an example, contribution to the processing(the calculation processing or the synthesizing processing of the secondbeamforming coefficient) of the signals of the spherical wave may be setto be smaller compared to the contribution to the signals of the planewave. In this manner, the signals of the spherical wave can also beutilized so that a signal from the wider region can be attained as theprocessing target, and the plane wave and the spherical wave aredifferently treated so that the effect of the adaptive beamforming suchas the MVB method can be obtained.

However in this case, the phasing processing becomes a problem. Asdescribed above, as long as the geometric models indicating propagationof waves are different from each other between the plane wave and thespherical wave, the methods of calculating the delay time are alsodifferent from each other. In the above-described embodiment, since nosignal of the spherical wave appears in the successive processing, thereis no occurrence of a problem even though the phasing processing whichis uniformly targeted on the plane wave is performed. If the signals ofthe spherical wave are used in the successive processing, the phasingprocessing needs to be separately performed between the plane wave andthe spherical wave.

Specifically, when it is determined that the signal processing targetpoint exists in the plane wave propagation region, the phasingprocessing unit 132 performs the first phasing processing which is theprocessing for plane waves, and when it is determined that the signalprocessing target point exists in the spherical wave propagation region,the phasing processing unit 132 performs the second phasing processingwhich is the processing for the spherical wave. Herein, the firstphasing processing is the phasing processing performed based on thepropagation time of the plane wave which is obtained through the planewave propagation model, and the second phasing processing is the phasingprocessing performed based on the propagation time of the spherical wavewhich is obtained through the spherical wave propagation model.

As described below with reference to FIGS. 25A to 26, it is possible toconsider two types of the spherical wave propagation model such as atransmission wave model and a reflected wave (a reception wave) model.However, herein in a narrow sense, the spherical wave propagation modelindicates the transmission wave model. The propagation time of the planewave is a transmission propagation time t_(emt) of the plane wave whichis obtained through the following Expression 6. However, the propagationtime thereof may be t_(emt) and the propagation time which is obtainedthrough the following Expression 4 (the total propagation time).Similarly, the propagation time of the spherical wave is thetransmission propagation time t_(emt) of the spherical wave which isobtained through the following Expression 10. However, the propagationtime thereof may be t_(emt) and the propagation time which is obtainedthrough the following Expression 4.

As described above with reference to FIGS. 1A and 1B, even though areflected wave (an ultrasonic echo) is reflected from one signalprocessing target point, since the propagation path R varies inaccordance with the position and the like of the element, the receivingtiming of the reflected wave varies. The phasing processing isprocessing for reducing (in a narrow sense, eliminating) deviation ofthe timing thereof, that is, deviation of the phase in waveforms, asillustrated in FIG. 3. In other words, if it is possible to specify atwhich timing the reflected wave from the signal processing target pointwhich is the processing target in each of the elements is acquired, thephasing processing can be performed. Specifically, the time taken by awave emitted from the element array for being propagated to the signalprocessing target point and the time taken for being propagated from thesignal processing target point to each of the elements may be specified.Specifically, the propagation time t_(ToF) is obtained through thefollowing Expression 4, and the signal at the timing corresponding tothe propagation time t_(Tof) is sampled among the reception signals s.

t _(ToF) =t _(emt) +t _(rev)  (4)

In the above Expression 4, the factor t_(emt) represents thetransmission propagation time which is the time taken by thetransmission wave emitted from the element array for being propagated tothe signal processing target point, and the factor t_(rev) representsthe reception propagation time which is the time taken by the reflectedwave (the reception wave) for being propagated from the signalprocessing target point to each of the elements.

The factors t_(emt) and t_(rev) can be obtained by using a geometricmodel of wave propagation. Among thereof, in regard to t_(emt), theusing geometric model needs to vary in accordance with whether thesignal processing target point exists in the plane wave propagationregion or exists in the spherical wave propagation region. When thesignal processing target point exists in the plane wave propagationregion, the plane wave propagation model which is the geometric modelindicating how the plane wave is propagated is used, and when the signalprocessing target point exists in the spherical wave propagation region,the spherical wave propagation model which is the geometric modelindicating how the spherical wave is propagated is used. Regardless ofwhether the transmission wave is the plane wave or the spherical wave,the reflected wave of the transmission wave which is reflected from thesignal processing target point may be considered to be propagated as thespherical wave having the signal processing target point as a point wavesource. In other words, in regard to t_(rev), the spherical wavepropagation model may be used regardless of the result of the regiondiscrimination processing.

Hereinafter, an example of obtaining t_(emt) by the plane wavepropagation model, an example of obtaining t_(emt) by the spherical wavepropagation model, and an example of obtaining t_(rev) by the sphericalwave propagation model will be individually described. Hereinafter,while being on the condition of the time t=0, it is defined that a waveis generated from the position (the center of the element array) of x=0and z=0.

FIG. 24 illustrates an example of the plane wave propagation model. Inthe plane wave, the wave surfaces in the same phase become line segmentsperpendicular to the transmission direction DR3, and the line segmentsmove in the direction of DR3 due to a speed c of the ultrasonic wave.Here, the coordinates of the signal processing target point r_(m) areset as r_(m)=(r,θ) in the polar coordinates. As described above, since awave is generated from the center of the element array at t=0, thearrival position of the wave at t=0 becomes the line segment indicatedby B1 in FIG. 24. Since the signal processing target point r_(m) ispositioned on the line segment at the timing when the plane wave arrivesat the signal processing target point r_(m), the arrival position of thewave at the aforementioned timing becomes the line segment indicated byB2 in FIG. 24. In other words, when the plane wave arrives at the givensignal processing target point r_(m), the plane wave is propagated asfar as a transmission wave propagation distance indicated by d_(emt).

Here, since the coordinates of r_(m) is (r,θ), and the transmissionangle is α, the transmission wave propagation distance d_(emt) can beobtained through the following Expression 5. When the speed c of theultrasonic wave is used, the transmission propagation time t_(emt) insuch a case is obtained by the following Expression 6.

d _(emt) =r cos(α−θ)  (5)

t _(emt) =d _(emt) /c  (6)

Subsequently, FIG. 25A illustrates the spherical wave propagation modelfor the transmission wave. According to the model in FIG. 25A, thespherical wave having the first element or the Nth element as a wavesource is propagated in the spherical wave propagation region. In thiscase, the first element is positioned at the end portion (the apertureend) of the element array and the Nth element is positioned at the endportion on the other side thereof. Waves are also output from otherelements included in the element array. However, the waves are mutuallycancelled in the spherical wave propagation region so that it is enoughto be considered that the intensity is small compared to the wave fromthe end portion. In other words, in consideration of the spherical wavehaving the element at the end portion as the wave source, it is possibleto obtain sufficiently accurate t_(emt).

Here, when the coordinates of the element at the end portion are (x_(i),0), the spherical wave arrives at the signal processing target pointr_(m) while having (x_(i), 0) as the wave source. Accordingly, thetransmission wave propagation distance d_(emt) in such a case can beobtained through the following Expression 7 by applying the cosinetheorem as illustrated in FIG. 25A.

d _(emt)=√{square root over (r ² +x _(i) ²−2rx _(i) sin θ)}  (7)

It is possible to obtain the time taken by the spherical wave outputfrom (x_(i), 0) for arriving at the signal processing target point r_(m)by dividing d_(emt) by the speed c. However, herein, a wave is generatedfrom the center of the element array at t=0 as described above. Sincethe transmission processing unit 110 performs processing for outputtingthe plane wave at the transmission angle α, the arrival position of theplane wave at t=0 is in a state of the line segment indicated by C1 inFIG. 25B. In order to realize the wave surface indicated by C1, theelement at the end portion needs to start driving at the timing prior tot=0. Specifically, in the element array, the element on the left sidefrom the center in FIG. 25B needs to be driven at the timing prior tot=0, and the element on the right side from the center needs to bedriven at the timing later than t=0. In other words, in order for thetransmission angle α to take an angle other than zero degrees, the drivetiming of each of the elements needs to vary in accordance with thetransmission angle. In other words, since the element at the end portionis driven at the timing different from t=0 excluding a case of α=0,t_(emt) cannot be simply obtained by dividing the above Expression 7 bythe speed c. Therefore, an offset time t_(offset) indicating deviationof the timing has to be reflected therein.

In order to realize the wave surface indicated by C1 in FIG. 25B, thewave output from the element at the end portion has to be propagated bythe distance indicated by d_(offset) as illustrated in FIG. 25B at thestage of t=0. Since the distance d_(offset) can be obtained through thefollowing Expression 8, the offset time is obtained through thefollowing Expression 9.

d _(offset) =x _(i) sin α  (8)

t _(offset) =d _(offset) /c  (9)

The transmission propagation time t_(emt) in a case where the sphericalwave propagation model is used by applying the results of the aboveExpressions 7 and 9 can be obtained through the following Expression 10.In FIG. 25B and the like, since the right direction of the drawing isconsidered as the x-axis forward direction, d_(offset) and t_(offset)obtained through the above Expressions 8 and 9 are negative values.Accordingly, the transmission propagation time t_(emt) is lessened byadding t_(offset) in the following Expression 10.

t _(emt) =d _(emt) /c+t _(offset)  (10)

FIG. 26 illustrates the spherical wave propagation model of thereflected wave (the reception wave). As described above, the sphericalwave may be considered for the reflected wave having the signalprocessing target point as the point wave source. Therefore, the signalprocessing target point and the straight line distance of the targetelement may be considered for the reception propagation distanced_(rev). When the values of the coordinates of the element areascertained, the calculation can be easily performed. In regard to thereception propagation time t_(rev) as well, the obtained d_(rev) may bedivided by the speed c.

According to the processing described above, it is possible to performthe phasing processing based on the result of the region discriminationprocessing. In a case where applying a signal of the spherical wave whenobtaining the second resolution signal s′, the phasing processingdescribed in the modification example may be performed as the phasingprocessing for the prior processing thereof.

Hereinbefore, the embodiment is described in detail. However, it ispossible for those skilled in the art to easily understand that newadditions and various modifications without substantially departing fromthe invention can be made. Therefore, all the modification examples areconsidered to be included in the scope of the invention. For example, aterm which has been disclosed at least once together with alternativeterm used in a wider sense or similar sense in this specification andthe drawings can be replaced with the alternative term at any place inthis specification and the drawings. The configurations and operationsof the ultrasonic measurement apparatus, the ultrasonic diagnosticapparatus, and the like are not also limited to those described in theembodiment. Therefore, various modifications can be executed.

The entire disclosure of Japanese Patent Application No. 2014-221046filed on Oct. 30, 2014 is expressly incorporated by reference herein.

What is claimed is:
 1. An ultrasonic measurement apparatus comprising: atransmission processing unit that performs processing for transmittingan ultrasonic wave at a given transmission angle; a reception processingunit that performs reception processing of an ultrasonic echo withrespect to a transmitted ultrasonic wave; and a processing unit thatperforms processing with respect to a reception signal from thereception processing unit, wherein the processing unit obtains aplurality of first resolution signals by synthesizing a plurality of thereception signals based on a first beamforming coefficient, and obtainsa second beamforming coefficient for synthesizing a second resolutionsignal having high resolution compared to the first resolution signalfrom the plurality of first resolution signals based on whether a signalprocessing target point belongs to a plane wave propagation region inwhich the ultrasonic wave is propagated as a plane wave or belongs to aspherical wave propagation region in which the ultrasonic wave ispropagated as a spherical wave.
 2. The ultrasonic measurement apparatusaccording to claim 1, wherein the processing unit selects a coefficientcomputation first resolution signal from the plurality of firstresolution signals based on a result of region discrimination processingregarding whether the signal processing target point belongs to theplane wave propagation region or belongs to the spherical wavepropagation region, and obtains the second beamforming coefficient basedon the selected coefficient computation first resolution signal.
 3. Theultrasonic measurement apparatus according to claim 2, wherein theprocessing unit synthesizes the selected coefficient computation firstresolution signal and generates the second resolution signal based onthe obtained second beamforming coefficient.
 4. The ultrasonicmeasurement apparatus according to claim 1, wherein the plane wavepropagation region and the spherical wave propagation region are regionsdifferent from each other in accordance with the transmission angle ofthe ultrasonic wave in the transmission processing unit.
 5. Theultrasonic measurement apparatus according to claim 1, wherein theprocessing unit performs region discrimination processing regardingwhere the signal processing target point exists in any one of the planewave propagation region and the spherical wave propagation region. 6.The ultrasonic measurement apparatus according to claim 5, furthercomprising: a storage unit that stores table data in which informationindicating where the signal processing target point exists in any one ofthe plane wave propagation region and the spherical wave propagationregion is caused to correspond to the given signal processing targetpoint for each transmission angle of a plurality of the transmissionangles of the ultrasonic wave from the transmission processing unit,wherein the processing unit performs the region discriminationprocessing based on the table data.
 7. The ultrasonic measurementapparatus according to claim 5, wherein the processing unit performs theregion discrimination processing based on a first direction in which afirst aperture end among apertures respectively provided with aplurality of ultrasonic transducers transmitting the ultrasonic waveleads to the signal processing target point, a second direction in whicha second aperture end different from the first aperture end among theapertures leads to the signal processing target point, and thetransmission angle of the ultrasonic wave.
 8. The ultrasonic measurementapparatus according to claim 5, wherein the transmission processing unitperforms processing for transmitting first to Kth (K is an integer equalto or greater than 2) ultrasonic waves at first to Kth transmissionangles, and wherein the processing unit performs the regiondiscrimination processing for discriminating where the signal processingtarget point exists in any one of an ith (i is an integer of 1≦i≦K)plane wave propagation region and an ith spherical wave propagationregion corresponding to an ith ultrasonic wave based on an ithtransmission angle and a position of the signal processing target point.9. The ultrasonic measurement apparatus according to claim 1, whereinthe reception processing unit performs reception processing of theultrasonic echo with respect to a transmitted ultrasonic wave in firstto Nth ultrasonic transducers, and wherein the processing unit performsphasing processing with respect to first to Nth reception signalsrespectively corresponding to the first to Nth ultrasonic transducers,synthesizes the first to Nth reception signals after phasing processingbased on the first beamforming coefficient, and generates the firstresolution signal.
 10. An ultrasonic diagnostic apparatus comprising:the ultrasonic measurement apparatus according to claim
 1. 11. Anultrasonic diagnostic apparatus comprising: the ultrasonic measurementapparatus according to claim
 2. 12. An ultrasonic diagnostic apparatuscomprising: the ultrasonic measurement apparatus according to claim 3.13. An ultrasonic diagnostic apparatus comprising: the ultrasonicmeasurement apparatus according to claim
 4. 14. An ultrasonic diagnosticapparatus comprising: the ultrasonic measurement apparatus according toclaim
 5. 15. An ultrasonic diagnostic apparatus comprising: theultrasonic measurement apparatus according to claim
 6. 16. An ultrasonicdiagnostic apparatus comprising: the ultrasonic measurement apparatusaccording to claim
 7. 17. An ultrasonic diagnostic apparatus comprising:the ultrasonic measurement apparatus according to claim
 8. 18. Anultrasonic diagnostic apparatus comprising: the ultrasonic measurementapparatus according to claim
 9. 19. An ultrasonic measurement methodcomprising: processing for transmitting an ultrasonic wave to a targetobject; performing reception processing of an ultrasonic echo withrespect to a transmitted ultrasonic wave; obtaining a plurality of firstresolution signals by synthesizing a plurality of reception signalsreceived through the reception processing based on a first beamformingcoefficient; and obtaining a second beamforming coefficient forsynthesizing a second resolution signal having high resolution comparedto the first resolution signal from the plurality of first resolutionsignals based on whether a signal processing target point belongs to aplane wave propagation region in which the ultrasonic wave is propagatedas a plane wave or belongs to spherical wave propagation region in whichthe ultrasonic wave is propagated as a spherical wave.